Unix or Linux? (n00b alert)

Ok, I have just started learning how to hack and several websites have recommended Linux or Unix. I kind of know what Linux is, but I know NOTHING about Unix. In regards to Linux, I have heard to use Ubuntu as a beginner. Also, I have Windows 7 on my computer and don't know how to add another OS without also adding a new hard disk. I saw that you could put them on a CD and select boot from disk and it would run independently, but I have no idea how. Thanks for helping a noob hacker just learning the skills.

Ubuntu can share the partition with another OS such as Windows.And it can boot from USB ,you can use it in the livecd mode,however,the performance isn't very well.Besides,Puppy linux is a small and efficient distro of Linux and can be run in RAM without installing it(something like that...I'm not very sure).